The present invention relates to dissipation of heat generated at the electrode tips and in the arms of electric spot welding guns. More particularly, the invention relates to construction of electric welding guns to provide for heat removal during welding operations without requiring a continuously supplied cooling source.
Resistance spot welding utilizes the flow of electricity to permanently join two or more overlapping metallic workpieces to one another. Typically, the metallic workpieces are placed between two opposing electrodes of a spot welding system gun assembly. The electrodes are then forced together until their tips contact the outer surfaces of the workpieces at a pressure sufficient to sandwich the workpieces and ensure an adequate electrical contact between the electrodes and the workpieces. Then an electrical current is induced to flow from one electrode tip to the other electrode tip by way of the sandwiched workpieces. The workpieces act as conductors in the resulting electrical circuit, and resistance to the flow of electrical current at the interfaces between the metals generates heat. The affected metal of each workpiece selectively becomes molten, and interacts with molten metal of an adjacent workpiece to form a weld nugget that permanently bonds the workpieces together at the point of electrode tip contact. Additional heat is generated due to contact resistance between the electrode tips and the work piece, as well as by joule heating in the arms themselves. The heat generated must be dissipated to avoid thermal overload and subsequent gun malfunction in production applications.
A number of factors relate to the creation of a weld nugget, including the force and area of contact between the electrode tips and workpieces, the level of current flow, the length of time that the current flow lasts, degree of workpiece imperfection, and even the condition of the electrode tips themselves.
The prior art teaches the importance of creating an adequate weld nugget. Therefore, spot welding systems are over-configured to generate a weld nugget even if there are significant workpiece imperfections by having high force, current levels, and current application times. Yet, many resulting welds are still imperfect. Therefore, typically, somewhere on the order of approximately one quarter of all welds in a workpiece are added to insure adequate structural integrity.
Further, such overcompensation for possible workpiece imperfection results in significantly higher deformation (mushrooming) of the electrode tips at the point of contact between the tips and the mating workpieces. If the electrode tips are inadequately cooled, the electrodes experience excessive tip wear, deformation, tip sticking and even tip melting, all of which contribute to poor weld quality and increased equipment maintenance. The generation of significant heat at the electrode tips also results in significant heat built up in the welding control unit, transformer, and secondary (i.e., high current) cable disposed between the electrodes and the transformer.
Moreover, the application of continuous significant electrode force upon the sandwiched workpieces requires the use of significant sources of compressed air. The compressed air provides for the actuation of various air cylinders to position the welding gun electrodes with respect to the workpieces to be sandwiched therebetween and to generate force.
The use of complex air and water cooling systems with their multitude of hoses and corresponding pipes, valves, and the like, in combination with the controllers and supply mechanisms, greatly increases manufacturing expense. It has been estimated that somewhere on the order of approximately one quarter to one half of the total cost of a spot welding system can be attributed to the use of air and water-cooling circulation systems. It is known to provide alternatives to air-actuated cylinders to position the weld gun, thereby eliminating come of the hoses. However, motor actuated or electrically actuated weld guns produce additional heat that must be removed from the weld gun assembly.
To complicate matters, typical spot welding systems must be custom designed, built, and tested, requiring the services of numerous skilled trades. Following initial construction and testing, the verified welding systems are then torn-down, transported, and rebuilt at a manufacturing facility where they will actually be used. Such intermediate steps significantly increase the time lag and cost in providing a complete electric weld system. Moreover, both the design and testing facility, as well as the final manufacturing facility, must make significant capital and continuous monetary investments in air and water-cooling circulation systems, customizing them for each individual spot welding system.
Nor is the complexity and cost limited to manufacturing of a spot welding system itself. The ongoing maintenance problems of requiring significant water-cooling and air circulation systems are extensive. It has been estimated that on the order of eighty percent of the down time of a typical spot welding system may be attributed to the host of air hoses, and feed and return cooling water hoses in combination with the corresponding pipes, valves, and other components.
There are additional costs to requiring complex water and air supply circulation systems. Each spot welding system becomes unique. Each length of hose, each bend in a pipe or conduit, and each selected placement for various cooling water fittings is necessarily tailored to the particular welding system. The kinematics of the host of hoses (pejoratively referred to as xe2x80x9cspaghettixe2x80x9d) cannot be accurately predicted or modeled. Accordingly, the robot movements in each robotic work cell must be inputted on-site, step-by-step, to ensure that hoses do not become entangled. To further exacerbate the problem, the resulting xe2x80x9cwindowxe2x80x9d in which a robot arm may move to reach, for example, a weld point, is significantly reduced, again due to the proliferation of the compressed air and water hoses and associated components. Thus, the time to program a robot arm is extensive. Even after programming, the resulting process time to process workpieces is often significantly increased by having a small movement window.
A multitude of factors goes into the design of a spot welding gun. For a given force that needs to be exerted by a weld gun, the factors that enter into the design of the weld gun include the strength of the actuator necessary to effect the weld, the speed with which the actuator can close the arms of the weld gun, the force that the actuator can exert on those arms at a speed commensurate with the desired output of the device, and the speed with which the device can create any particular weld and then move on to the next weld to be performed. The speed with which the weld gun can dissipate the heat generated during the welding process is a contributor to this final factor. Should the weld gun become overheated, the weld tips can become damaged or the weld gun can get out of alignment due to warping. Materials being welded can also be more susceptible to high temperatures generated during the welding process.
A weld gun designer traditionally had only two options available in keeping the temperatures of the welding apparatus within tolerances. The first is to extend the cycle times such that the apparatus has time to cool, thereby keeping the apparatus temperature down. The second is to provide a structure whereby the apparatus is cooled either continually or cyclically during the manufacturing process. As noted above, such a cooling mechanism, however, adds drastically to the cost of the apparatus and to the cost of operation, and the extra accoutrements that go with such an apparatus can hinder the flexibility and mobility of the apparatus.
Accordingly, an all-electric resistance welding system is desired that eliminates the need for extensive water fittings or other continuously available cooling apparatuses to simplify the construction, installation and maintenance of the welding system, and to improve the quality and reliability of a weld.
Heat generated at the electrode tips or in other areas of electric welding guns, such as in the welding control unit, transformer, and secondary (i.e., high current) cable disposed between the electrodes and the transformer, is dissipated through a combination of inherent internal, external and structural features. The features may be used alone, or more preferably, are used in combination to maximize heat removal without the need for a continuously renewed cooling mechanism such as water flow. In general, a weld gun includes a pair of opposed electrode tips. In a first assembly, each electrode tip includes an internal coolant reservoir containing a predetermined amount of coolant. A heat tube interconnects each reservoir to a location remote from the electrode tip. In operation, the coolant in each reservoir vaporizes as it absorbs heat generated at the tip. The resulting heated vapor travels through the beat tube to the remote location, where heat is removed from the vapor, causing the vapor to condense back to liquid. The liquid is then returned to the reservoir. In one embodiment, the liquid is directly and continuously returned to the reservoir through the heat tube, thereby creating a closed loop cooling system. In another embodiment, the condensed liquid returns to a holding tank that interconnects with the reservoir. In this embodiment, a dose of liquid coolant is supplied to each reservoir at predetermined times in a welding cycle to ensure that sufficient liquid remains in the reservoir to adequately cool the electrode tip. The dose is supplied as the liquid in the reservoir depletes due to vaporization of the liquid as it absorbs heat.
In a second assembly, the electrode tip is provided with a heat capacity capable of storing the heat generated during a weld cycle of a predetermined length, wherein a weld cycle is one or more welds made on a work piece. At the end of the weld cycle, the electrode tip is interconnected to an external cooling device that directly conducts stored heat away from the heat capacity, thereby directly removing the heat from the electrode tip. The external cooling device may also include secondary cooling mechanism to dissipate the heat conducted away from the heat source, thereby increasing heat capacity. The external heat sink may be shaped to receive electrode tips of the weld gun, or it may be moved to conform with and contact an exterior surface of the weld gun.
In a third assembly, the electrode tip is interconnected structurally to a cooling mechanism such that heat generated at the weld tip is conducted through the electrode tip structure to a secondary cooling mechanism that is cooled by convection to ambient air. In one form, the secondary cooling mechanism includes external fins. The fins may be used in combination with the heat tube of the first assembly, or may be formed directly adjacent the device tip. In a preferred form, the weld gun arm is constructed from a number of thin individual plates spaced to allow air convection cooling between them. During a weld cycle, heat is dissipated from the electrode tip back to the plates of the weld gun arm by natural conduction or other means, and is further transferred to ambient air by convection cooling from the plates. The plates are joined at connection points to maintain spacing and may include cross pins to provide buckling strength. By designing and building the weld gun arm itself from multiple plates, more surface area is provided for cooling without excessive machining of material. The plates provide both structural support for the weld tip while also acting as fins for the dissipation of heat generated at the tip. Importantly, the plates are substantial enough to support the weld tips during application of electrode force upon work pieces sandwiched between the tips. Additionally, the weld gun arm is lighter in weight than conventional arms and requires less material to form.
The heat removal assemblies of the present invention offer several significant advantages over conventional cooling methods. The heat removal assemblies are part of an integrated welding system, thereby eliminating the need for complex customization of the cooling system. An integrated system also eliminates the maintenance costs associated with custom designed water and air-cooling systems that have been necessary for the operation of electric weld guns. Further, by including the cooling system with the electric welding gun, the stability of the cooling system is no longer a rate-limiting step. In fact, the combination of cooling systems disclosed herein allows for optimization of the systems before installation. Thus, the rate of failure for a welding apparatus having an integral cooling system is substantially lower than that of an continuously supplied, customized cooling system. Additionally, the welding systems of the present invention may be implemented in areas where external supplies of cooling water are limited or are non-existent, since the cooling systems are integral with the welding guns.